1. Field of the Invention
The present invention relates to a thin-film magnetic head, and more particularly to a thin-film magnetic head which exhibits a reduced nonlinear transition shift when used for high-density high-frequency digital recording, to thereby enable the high-density high-frequency digital recording.
2. Description of the Related Art
The recording density of magnetic recording devices has been drastically increased, and in the field of magnetic disk drives for computers, the recording density has increased at a rate of 60% per year. In order to further increase the recording density of magnetic recording devices, magnetic recording media must be improved. In addition, there must be developed a thin-film magnetic head that provides excellent recording/reproducing characteristics when applied to such improved media.
For example, an MR (magnetoresistance effect type) head--which provides a considerably higher output as compared with a conventional induction type head--has been developed and put into actual use for a reproducing head in order to achieve a recording density of 1 GB per square inch or more. Meanwhile, a conventional induction type thin-film magnetic head--which utilizes electromagnetic induction--has been used for a recording head. A recording/reproducing thin-film magnetic head in which the above-described reproducing head and recording head are integrally formed has been widely used.
FIGS. 10A-10C show the structure of an MR composite-type thin-film magnetic head presently used. FIG. 10A shows a top view of the MR composite-type thin-film magnetic head. FIG. 10B is a cross-sectional view taken along line A-A' in FIG. 10A, showing the vicinity of a floating surface. FIG. 10C is a cross-sectional view taken along line B-B' in FIG. 10A, showing the floating surface. In the MR composite-type thin-film magnetic head, a lower magnetic pole 2, an insulating layer 5, and an intermediate magnetic pole 3 are formed on a substrate 1, and a reproducing section 9 having an MR film 6 is formed between the lower magnetic pole 2 and the intermediate magnetic pole 3. A recording gap layer 10 is formed on the intermediate magnetic pole 3, and a coil 11 is formed on the recording gap layer 10. In order to insulate the coil 11, there is formed an insulating layer 12, on which an upper magnetic pole 13 is further formed. Further, a protective layer 17 is formed as an uppermost layer for protecting the above-described elements.
For the recording magnetic pole of the above-described thin-film magnetic head, there has conventionally been used a plated film (film formed through plating) of 83 Ni-17 Fe (wt. %) alloy. This material has excellent soft magnetic characteristics, and can be made into film through plating. Therefore, this material is suitable for a magnetic pole of a thin-film head having a narrow track width.
However, in the above-described thin-film magnetic head, there have arose the following problems as recording density and recording frequency have increased.
In magnetic disk drives for computers, with an increase in track recording density, characteristics in terms of overwriting (O/W) and nonlinear transition shift (hereinafter abbreviated as "NLTS" ) become unsatisfactory. Therefore, improving the O/W and NLTS characteristics is an important theme for coping with a future increase in recording density. NLTS is a phenomenon which occurs in relation to recording of digital signals and in which a signal is recorded at a position shifted from the position where the signal is to be recorded. O/W is a phenomenon which occurs when a signal having a shorter wavelength is written in a superposed manner after recording of a signal having a longer wavelength and in which the signal having a longer wavelength remains unerased. O/W becomes a great problem when a recording medium having a high coercive force is used, and when the intensity and gradient of a recording magnetic field are insufficient for the recording medium. NLTS occurs when the high-frequency characteristics of a magnetic pole become insufficient during high-frequency high density recording and therefore delay occurs in variations in the recording magnetic field.
In order to increase the recording density of a recording medium, it is necessary to increase the coercive force of the recording medium to thereby prevent demagnetization due to a opposing magnetic field of a recording magnetization, and it is also necessary to decrease the size of a magnetization transition region, to thereby enable to an increased output to be obtained when recording is performed at high density. However, when a recording medium having a high coercive force is used, a strong recording magnetic field is required for sufficient magnetization of the recording medium in accordance with a signal. The above-described 83 Ni-17 Fe (wt. %) alloy having a saturation magnetic flux density of about 1 T causes a problem of magnetic pole saturation when it is used for a recording medium having a coercive force of 190 kA/m or higher, resulting in insufficient strength of the recording magnetic field. The insufficiency in the strength of recording magnetic field appears as deterioration of O/W characteristic, so that the level of O/W, which must be -30 dB or less, increases to -26 dB or greater.
Meanwhile, concurrently with the increase in recording density, recording frequency has increased. For example, there has been developed a magnetic disk drive that uses a disk having a diameter of 3.5 inches and is operated at 80 MHz or higher. When such a high frequency is used, the speed of changes in the magnetic flux of a magnetic pole cannot follow the speed of changes in record signals, so that delay is produced in changes of the recording magnetic field, resulting in deterioration in the above-described NLTS characteristic.